Commercial colistin was separated into three major components as well as lipid components by reversed phase adsorption chromatography on Diaion HP-20 AG, a macroreticular styrene-divinylbenzene copolymer, without any inorganic salts or detergents, in aqueousorganic solvent as mobile phase. These expected components were colistins A, B and C; there were, however, appreciable differences between these components and colistins A, B and C, isolated by countercurrent distribution. The newly isolated components showed slightly higher potency than colistins A, B and C; their molecular weights, as determined by gel permeation chromatography of 2, 4-dinitrophenyl derivatives on TSK-GEL G2000H (mobile phase: dimethylformamide), were also slightly higher. Accordingly, they were tentatively named colistins pro-A, pro-B and pro-C. During purification by countercurrent distribution (solvent system: sec-BuOH-n-BuOH-0.1 N HCl, 30: 6: 40), colistin pro-A was converted to colistin A. Similarly, colistin pro-B was converted to colistin B, and colistin pro-C to colistin C. Therefore, we concluded that colistins A, B and C are artifacts.